Compatible with Waterfox! Very customisable and powerful add-on, allowing selecting a specific agent for each visited domain. It has two different interfaces, one more "user friendly" and another more "programmer friendly". One problem I noticed is, aside from PlayFlash add-on, which contains Adobe's FlashPlayer DLL inside, this is the biggest add-on on the bunch! A previous version was 8 MB in size (about 41 MB unpacked!!!), current is half of that (4 MB / 21 MB), all because of one single utility, "deps/browscap.js", which has a huge cache of hundreds (!!!) of files with what look like site "id" codes, which may be completely removed without breaking the add-on (just delete all the hundreds files from deps/browscap cache in the XPI file and install the add-on).
EDIT: I guessed the huge cache was for self identification (browser, OS etc.), but didn't take the time to check enough of the code. There could be a "light version" without that functionality. Other agent switching add-ons lack one or all the other features of this one, like domain specific selection, fully customizable agent strings and easy pop-up selection of strings. Anyway, thanks for replying! I appreciated the attention!